I'm new to the SLR world. I've been pouring over reviews, tips and lessons. But the one thing I can't find much advice on is ISO. I know it is a sensitivity setting but I can't seem to find many tips/advice on it other then pics get really grainy at really high ISO numbers and poor lighting.

Could someone please post some basic tips on pro's and cons of high or low ISO settings? Situations where you want a low ISO and situations where you want a higher one?

Baring this a link to a great tutorial of ISO would be just as welcome!

If you need the best quality, set a low value and make sure both the camera and subject stay still long enough, and/or throw enough light at it. In the real world, set the ISO as needed for balancing the other options. There is no right value. A low ISO for better quality is no use for example if everything is out of focus or blurry due to motion.

Thanks for the advice Popo. Here's a scenario. On a sunny day if I was shooting stills I would worry about my f number and shutter speed and keep my ISO around 100 - 200? If on the same sunny day I was shooting action shots I would again worry mostly about my shutter speed and f number but raise my ISO a touch to help reduce bluring?

Most people think of ISO as the last part of the exposure triangle to adjust - worry about shutter speed and aperture, and keep ISO on the very lowest setting. Only when you can no longer achieve the shutter speed you want at a given aperture would you then start raising the ISO to get the shutter speed you need. Some folks go for opening up the aperture more and more, and only raise ISO when they are wide open and can go no further - but there may be times when you need a smaller aperture for depth of field or sharpness and cannot achieve a fast enough shutter speed, so ISO can be raised as needed in those situations. In general, always use the least ISO you need to get the shot.

That makes it sound like I'm against high ISO shooting - which I'm not at all - in fact I regularly shoot up to ISo6400. But I always stick to the rule of adjusting the aperture and shutter first, and then the ISO if needed.

Advantage of raising the ISO is the increased sensitivity of the sensor in low light situations or where you need more light to get a faster shutter speed. Disadvantage of higher ISO is the increase in noise/graininess in the shot as the ISO goes higher.

Think of ISO like an old 'gain' dial on a HAM or shortwave radio, or CB, or maybe like the 'brightness' dial on a TV or monitor. You turn it up to increase the sensitivity to get a signal or brightness as needed, but you notice with 'gain' that the noise and static increase as you go higher, or with brightness the contrast fades and becomes overwhelmed by whiteness. ISO is similar, in that you get more sensitivity to pull in light, but deal with more graininess and noisy pixels the higher you go. The goal is to find the perfect compromise between just enough sensitivity to get the shot, while as little as possible to keep the noise and grain to the minimum.

Unless you are specifically going for the kind of grain that higher ISO may give (and you don't want to use a software grain-engine in post-processing), I second zackiedawgs advice.

Regardless of the drastic improvements over the last few years, in high-ISO noise control, the premise remains the same. You are electronically amplifying a signal - much like turning up the volume on a stereo system. It's an electronic adjustment that has nothing to do with the path of the light.

The trick is to be conscious about it - For example: if you have F8 at 1/60sec at ISO 800, F11 at 1/60sec require ISO 1600. Conversely, to achieve 1/120 sec at F8 will require ISO1600.

Here's what I do - I tell my auto-ISO that I want to use minimum ISO200 and max ISO1600 and then I set a minimum shutter speed @ 1/30sec (which is about the slowest I can handhold a 35mm lens on a DX camera without stabilization) and then I use shutter or aperture priority as desired. With a different lens, I may change the minimum shutter speed accordingly.

If we are talking human subjects moving "normally" (i.e not all-out sports, but goofing around and even dancing - except if it's heavy metal or dubstep..hehe) the required shutter speed is 1/250sec to freeze motion (give and take 25%). For athletic endeavors and running dogs, it it more 1/sec.

if you have such thumb-rules in mind and make your preferred settings accordingly, you can ensure that you achieve lowest possible ISO under a range or circumstances.